Shuttle valve will have three port i.e. one common
output port and there will be two inlet ports. Figure shows the shuttle valve,
source 1 and source 2 are inlet ports and there is also one port which is
outlet port.

Shuttle
valve working principle

Let pressure at source 1 is P1 and pressure at source
2 is P2. Suppose, pressure P1 is higher than the pressure P2, ball will slide
towards the secondary port and will block the source 2. Hydraulic fluid will
flow from source 1 to outlet port. If pressure P2 is higher than the pressure
P1, ball will slide towards the primary port and will block the source 1. Hydraulic
fluid will flow from source 2 to outlet port.

It indicates that shuttle valve outlet port will
secure the fluid flow from its inlet port which will be at high pressure. Shuttle
valve applications could be seen in load sensing and will secure the flow from
higher pressure inlet out of its two inlet port of shuttle valve.

Internal cross section of shuttle valve is displayed
here and symbol used for indicating the shuttle valve is also displayed here in
following figures.